Through the 300 million years that trilobites existed,
prior to their extinction
in the Permian, there were many opportunities for diversification of form,
starting from the presumed primitive morphology exemplified by a
species such as Redlichia (left). This typical primitive morphotype
had a small pygidium, well developed eye ridges, a simple, lobed glabella,
several thoracic segments, and a rather flattened body form.The first trilobites were characterized by this primitive
form. Among the over 20,000 species of described trilobites there are species
in which aspects of morphology have diverged greatly from the primitive state.
These are discussed in Fortey & Owens 1997 (see citations below). Thoracic
segments were reduced to as few as two or increased to
over 100, overall body shape was greatly elongated in
some, or rendered transverse (widened) in others. Examples are shown
below:

Shapes and furrow patterns of the glabella, and the shape
and placement of eyes and eye ridges of course also ranged widely.
An analysis of morphological diversity of trilobite forms showed that increasing
from the Cambrian, there was a peak in morphological diversity
in the Ordovician
(which parallels a peak in overall diversity of trilobites families) that
decreased only as overall trilobite diversity decreased toward their extinction
in the late Permian.

Within this diversification, there were a number of evolutionary
trends in morphology that developed in unrelated clades, creating
homeomorphy (attainment of similar forms in unrelated groups).
These homeomorphic trends, such as effacement, increased spinosity, reduction
in body size, streamline shape, and loss of eyes, can not be reliably or
consistently used to assess higher systematic relationships. Instead, these
features can tell us about selective pressures on trilobites
and how similar solutions were derived in parallel by different evolutionary
lineages. Each of these is discussed below, and examples are given from different
orders.

Illaenus atavus

Effacement
In several trilobite orders, but perhaps most notably among the Agnostida, Corynexochida (Suborder
Illaenina), and Asaphida,
effacement of cephalic, pygidial, and even thoracic furrows is not uncommon.
This loss of surface detail can be confounding to systematics, since effaced
features (for example loss of glabellar details) can mask evidence of relationships.
Some workers suggest that effacement is an adaptation related to a burrowing
lifestyle, especially in Illaenina, but such effacement might also play
a role in streamlining of pelagic Asaphida, and is also seen in many Agnostida
(which may have been planktonic). No single selection pressure seems to
have been responsible for the effaced morphotype.

Asaphida Cyclopyge

Corynexochida Bumastus

Asaphida Nileus

Agnostida Lejopyge

Effaced trilobites, such as those above, can be difficult
to classify, since they lack key characters.

Leonaspis sp (Morocco)

Spinosity
The development of spines is commonly thought of as primarily a defensive
adaptation, and increased spinosity is seen in a wide variety of trilobite
species. Alternate hypotheses for the adaptive significance of spines include
stabilization structures on a loose silty substrate (e.g. genal
spines of trinucleid Asaphida, such as Ampyx, below), and flotation/stabilization
structures for slow-swimming taxa (e.g., odontopleuroid Lichida
such as Leonaspis at left). Spines may originate from just about any
part of the exoskeleton, especially the margins. Sometimes these patterns
provide consistent and diagnostic characters for higher classification (for
example, the pattern of pygidial spines among odontopleuroid Lichida). However, as seen in
the spinose species below, development of spines occurs in many orders
of trilobites.

Phacopida

Comura

Asaphida

Ampyx

Corynexochida

Oryctocephalus

Proetida

Phaetonellus

Spinose trilobites are a clear indication that it
was dangerous world in the Paleozoic, in which an unarmed trilobite might
likely be swallowed whole.Miniaturization
Reduction in size is seen in several trilobites, such as Acanthopleurella
(just about 1 millimeter at maturity). The general argument for evolution
of small size typically evokes the numerous microhabitats of complex marine
systems, coupled with a correlation of size to rapid maturity (early maturation
means smaller size at adulthood). When this reduction is due to progenesis
(arrested development) the trilobites may also display a reduction in the
number of thoracic segments (see section on Ontogeny). Thoracocare, for example, has only
two thoracic segments at maturity, yet is not a member of the order Agnostida
(formerly thought to be the only order bearing three or fewer thoracic
segments), but of Corynexochida. The entire order Agnostida is composed
of small species that may have originated as a miniaturized and specialized
early offbranch of the Redlichiida or Ptychopariida.

Ptychopariida Acanthopleurella

Corynexochida Thoracocare

Ptychopariida Shumardia

Agnostida Pagetia

Ptychopariida Schmalenseeia

.

In a complex marine environment, many niches (physical
and ecological) are available for small species.

Ellipsocephalus hoffi

Atheloptic MorphologyReduction and loss of eyes
in trilobites has been discussed elsewhere, and secondary reduction and
loss of eyes is thought to be a trend among benthic species living in
deep, poorly-lit or aphotic habitats. In these deep water biotopes,
blind or nearly-blind trilobites are the dominant element. Typically,
these atheloptic species have close relatives in which eyes are of
normal size and function. It
is interesting to note that another trend of deep bottom habitat
adaptation is an increase in the number and width of thoracic segments,
which might be related to specialized feeding adaptations (see
Olenimorph section below).

Ptychopariida Lermontovia

Ptychopariida Conocoryphe

Phacopida Trimerus

Atheloptic trilobites had little use for eyes on the
dark bottoms of ancient seas.

Reconstruction of Carolinites genacinaca

Pelagic Morphology
There are a number of trilobites that have developed extremely large
eyes and elongate, streamlined body shape associated with swimming in the
photic water column. The paleogeography
of some of these pelagic species (for example, Carolinites, shown
at left), suggests that their swimming abilities were good enough them to
spread into a global oceanic distribution.

Olenimorph
Thin exoskeleton, increased numbers of thoracic segments, and a widened,
flat body form is associated with benthic habitats marked by low oxygen
and high sulphur compound concentrations. Fortey suggests that olenimorphs
(so named because many of the Ptychopariida Suborder Olenina have this form) may represent
the first symbiotic relationships with sulphur-eating bacteria as a feeding strategy. The numerous transverse
thoracic pleurae presumeably overlaid a series of laterally extended gill
exites, maximizing oxygen absorption and providing a large surface
area upon (or within) which symbiotic bacteria could live.

Pitted Fringe
An expansion of the cephalon into a concave chamber perforated by
numerous fenestrations is thought to be an adaptation related to filter-feeding. This morph has arisen
in at least two unrelated groups: Trinucleioid Asaphida and members of the Order Harpetida. In both cases the cephalon is the dominant
part, with transverse thoracic segments and an unremarkable pygidium. Long
genal spines or genal prolongations are also notable in this morphotype.
They are thought to help stabilize the trilobite during filter-feeding.

Harpetida Harpes

Asaphida Cryptolithus

The pitted fringe trilobites are among the most distinctive
in appearence.References for this page